It is commen for young girls to gt their periods as early as 6/7yrs old. A friend has a daughter that went through the same. A duty doc or cover doc should be availible.
To be frank, there is no point speculating about this. It does no good. She should be evaluated. While use of the ER should be avoided except for true emergencies, if her primary care provider is not available it is reasonable to do so (unless there is a covering provider - ask about that). In the ER it is likely lab work will be ordered to help with the diagnosis.